PPM1A dephosphorylates CDK9, but not CDK8 in an artificial CDK tethering system. Triplicate HeLa cell cultures were transfected with pSLIIB-CAT reporter plasmid and expression plasmids for different effector proteins: CDK9-Rev, CDK8-Rev, PPM1A-WT, PPMIA-MT, Tat-Rev, and Rev as indicated. Cell extracts were prepared at 48 hours post-transfection and CAT activity was measured and normalized to total protein in extracts. (**denotes p value of < 0.005 in a paired t-test.).